1. Field of the Invention
This invention relates to systems for frequency doubling the output of a number of diode lasers and combining the frequency-doubled output to achieve high brightness. In particular this includes frequency doubling of the laser output from diode laser arrays such that the individual beams are added together to produce high quality, high brightness frequency-doubled light.
2. Information Disclosure Statement
Nonlinear optical devices, such as harmonic generators and parametric oscillators, provide a means of extending the frequency range of available laser sources. Frequency converted solid state lasers currently fall into two categories: high power devices based on conventional solid state lasers (pumped by either flash lamps or diodes) plus frequency conversion crystals or low power devices (microlasers) based on a diode with a combination of a laser crystal and a non-linear crystal attached to it. Experimentally, low power diodes have also been directly frequency converted.
For years many groups around the world have been working to develop diode lasers directly operating in the blue/green region of the spectrum, however, with little success. Another way to reach shorter wavelengths is to use nonlinear materials, which are able, for example, to combine two photons into one which has the added frequency of the individual photons. This technique provides a comprehensive range of new wavelengths and allows building highly efficient systems.
The generation of visible light via frequency doubling has attracted growing interest over recent years, owing to its potential use in high density optical storage and in medicine. Traditionally, frequency doubling of infra-red lasers has been accomplished with nonlinear crystals which rely on birefringent phase-matching. This dependence on birefringent phase-matching has greatly restricted the range of suitable nonlinear materials as well as the range of wavelengths that can be efficiently doubled. The net result has been that cw frequency doubling efficiencies in single-pass configurations have tended to be rather low.
More recently, there has been increasing interest in the use of quasi-phase-matched nonlinear crystals. Quasi-phase-matching (QPM) has several advantages over birefringent phase-matching, including access to higher nonlinear coefficients and non-critical interaction geometries for any wavelength in the transparency range of the crystal. QPM can be achieved by an appropriate periodic modulation of the nonlinear coefficient.
Nonlinear gratings fabricated in crystals such as KTiOPO.sub.4 (KTP), LiTaO.sub.3 and LiNbO.sub.3 have been used for blue light generation via frequency doubling both in bulk and waveguide geometries.
The output power of a single diode laser emitter is limited by the power density on the facet and the possibility to transport the generated heat away from the diode. To gain higher output power, diode laser devices consist usually of several emitters or emitter groups which are combined to an array of emitters on a substrate. This arrangement decreases the reachable beam quality in one direction by the number of the emitters. It limits the use in several application where a higher power density is necessary.
A measure of beam quality is beam propagation parameter, M.sup.2, which can be calculated and analyzed to determine how well a laser beam may be focused. Generally, the highest quality beam is associated with the highest focusability to the smallest spot size, which corresponds with the highest power density. FIG. 1 shows the variables used to determine M.sup.2, which is directly related to the product of a beam's minimum near-field diameter, W, and beam divergence angle, .theta., in the far field for a specific emission wavelength, .lambda.: EQU M.sup.2 =.pi.W.theta./4.lambda. (1)
Laser beams with M.sup.2 =1 are ideal, and larger M.sup.2 values indicate decreasing focusability of a laser beam; M.sup.2 values less than 1 are unattainable.
The near field is the region at, or very close to the output aperture of the diode laser emitter, which is characterized by disordered phase fronts, and is often called the Fresnel zone. In the near field, shape, size, profile and divergence can vary rapidly with distance along the beam path. The extent of the near field depends on the laser type and for a highly divergent source such as a diode laser, it can be as short as a few microns from the output facet. In contrast, the near field of an excimer laser might be many meters. To attain the highest brightness (power density) the laser beam needs to be captured at its smallest area, which, generally for a divergent beam, is as close as possible to the emitter.
At longer propagation distances from the laser, the phase fronts become ordered, leading to stable beam characteristics. This is known as the far field, or Fraunhofer zone. A very rough approximation of the distance to the onset of the far field region can be obtained by taking the square of the beam's minimum near field diameter, W, divided by the wavelength, .lambda.: EQU F=W.sup.2 /.lambda. (2)
For example, for a typical HeNe laser having a circular output (W=1 mm, .lambda.=632.8 nm), the distance to the far-field begins at about 1.5 m, while for a typical YAG laser having a circular output (W=10 mm, .lambda.=1064 nm), the far field distance begins nearly 100 m from the source.
The quality of a diode laser beam is typically examined with respect to the fast axis, which is the high divergence axis, perpendicular to the pn-plane of a semiconductor diode, and with respect to the slow axis, which is the lower divergence axis, parallel to the pn-plane of a semiconductor diode. FIG. 2 shows that emitted laser beam 24 from semiconductor laser diode 21 propagates along the z-axis and diverges rapidly along the y-axis, termed the fast axis, which is along the minor axis of diode laser stripe 23. Concurrently, emitted laser beam 24 diverges slower along the x-axis, termed the slow axis, which is along the major axis of diode laser stripe 23. Near the source, emitted laser beam 24 is elliptically shaped with the x-axis being the long axis. The minimum near field diameter, W, is therefore different along the fast and slow axes. For M.sup.2.sub.slow calculations, W is assumed to be the effective diameter along the major axis, or x-axis, and for M.sup.2.sub.fast calculations, W is assumed to be the effective diameter along the minor axis, or y-axis. Since the effective diameter, W, is so much larger along the slow axis compared to the fast axis, M.sup.2.sub.slow typically are larger than M.sup.2.sub.fast, indicating that beam quality is greater along the fast axis. M.sup.2 for a beam is equal to the square root of the product of the M.sup.2 values for both axes, i.e. M.sup.2 =(M.sup.2.sub.fast M.sup.2.sub.slow).sup.1/2.
As emitted laser beam 24 propagates away from the source, it diverges more rapidly along the y-axis than the x-axis. After some distance, laser beam 24 will be circular for an instant, and thereafter, the long axis of the ellipse becomes the y-axis. Generally, for many high power laser diodes, the fast axis diverges at about 40.degree. and the slow axis diverges at about 20.degree..
Adding diode laser emitters to form an array normally leads to an M.sup.2 value for the "enveloped" beam where "enveloped" is used to describe the combined beams of the diode laser emitters, and is given by: EQU M.sup.2.sub.enveloped &gt;.SIGMA.M.sup.2.sub.n (3)
where n is the number of emitters. M.sup.2.sub.enveloped increases as the effective width of the "enveloped" beam increases, and is greater than the sum of the M.sup.2 values for the individual beams because the effective width includes the space between the emitters. It thus would be advantageous to increase beam quality by employing suitable optics to combine the beams and limit the effects of the space between the emitters. If the beams can efficiently be combined, M.sup.2.sub.enveloped can be decreased, and therefore, greater power density and beam brightness is available at a work or treatment site. The minimum value of such an M.sup.2.sub.enveloped is still the sum of the individual M.sup.2.
Another limitation of currently used diode laser systems is the wavelength. Until now the green/blue spectral area has not been reliably achievable. On the other hand diodes in the near infra-red area are available and reliable.
Thus far the solutions to these limitations/problems are either complex and inefficient and/or the output power and output wavelength are very limited. It is the object of this invention to solve these problems.